Magnetic resonance imaging (MRI) with its lack of ionizing radiation, good sensitivity of deep tissue, rich tissue contrast, and high-resolution capability offers many advantages for anatomical and functional imaging of various organ systems from humans to mice. MRI is highly suited for imaging events at the cellular and subcellular levels.
The phase divided by the imaging echo time and the gyromagnetic constant is the local magnetic field of magnetic biomarkers. The local field is a convolution of a dipole field kernel with the biomarker's magnetization distribution. The inverse solution from field to source quantitatively determines the magnetization distribution or the concentration map for a given magnetic biomarker.
Molecular imaging is a rapidly emerging biomedical research discipline that includes the use of targeted or activatable reporter agents to sense specific molecular targets, cellular processes, or particular pathways. Quantification is essential for measuring and characterizing biological processes. While MRI is an important tool for molecular imaging, current relaxation-based quantification in molecular MRI has been problematic due to the complex relation between the detected MR signal intensity and the magnetic biomarkers.
Molecular imaging refers to in vivo visualization and measurement of biological processes at the molecular and cellular level using endogenous or exogenous biomarkers. Quantification of biomarkers or contrast agents is essential for molecular imaging to stratify disease and gauge therapy. MRI is a very useful modality for molecular imaging because it does not use ionizing radiation and offers unlimited depth penetration and high spatial resolution. However, current quantification of magnetic biomarkers in molecular MRI based on relaxation measurements is well known to be problematic: 1) a calibration is needed for absolute quantification, and 2) the assumption of spatially uniform availability of free water for relaxation (T1 and T2) with contrast agent breaks down when contrast agents become bound to cells and molecules as in molecular imaging. Other approaches for visualizing contrast agents in MRI using the negative susceptibility-relaxation T2* contrast or off-resonance phenomenon have not shown to be capable of quantification.
Quantification is essential in experimental methods used to study biochemical reactions, biomolecular pathways and biological processes in health and disease. The importance of quantifying molecular/cellular events cannot be overemphasized for molecular imaging. For example, the use of nanoparticles as delivery vehicles for diagnostic and therapeutic agents requires accurate counts of nanoparticles accumulated at the diseased tissue to make diagnostic decisions and gauge therapeutic dose. The measurement of drug dose at targeted sites is essential for monitoring therapy. The count of stem cells homing at diseased tissue would be essential in optimizing cell therapy protocols. The goal of in vivo study of biochemistry through imaging necessitates a means to quantify molecular events. Quantitative accuracy and reproducibility have to be established to standardize and cross-validate molecular MRI methods. So far there is no effective tool to quantify molecular/cellular events. Molecular MRI investigations have been only qualitative or semi-quantitative. Estimation of signal changes currently used in MRI, such as hypointensity in detecting SPIO labeled or targeted cells does not provide absolute quantification and may be highly dependent on imaging parameters, pulse sequences and field strengths. Absolute quantification of magnetic biomarkers will enable longitudinal investigations and inter- and intra-scanner evaluations that are essential to molecular imaging based diagnostics and therapeutics.
A specific example is to develop targeted cancer therapeutics that aims to kill tumors without damaging healthy tissue. The many possible drug interactions in a human system make it very difficult to increase the desired site specificity, and a noninvasive quantitative determination of drug biodistribution would be a very useful tool to guide the development of cancer targeting. Recently multifunctional polymeric micelles loaded with SPIO and doxorubicin for cancer targeted delivery have been developed. Current MRI techniques allow visualization but not quantification of SPIO.
Another example is to develop noninvasive MRI of gene expression. In vivo detection of gene expression using optical or radioactive reporters shows great promise in monitoring cell trafficking, gene replacement therapy, protein-protein interactions, neuronal plasticity, and embryonic development; however, overcoming tissue opacity and resolution limitations remains a key challenge. MRI reporters that generate contrast agents have been developed, providing high resolution deep tissue in vivo imaging and anatomic correlation. Current MRI techniques allow good visualization but not accurate quantification of gene expressed contrast agents due to limited free water for relaxation.
To explore biomedical applications, magnetic susceptibility measurements of biomaterials have been investigated using a superconducting quantum interference device (SQUID) and MRI signal phase. The basic approach for estimating an object's susceptibility is to polarize the object with a known primary magnetic field and measure the field associated with the magnetization of the polarized object. Maxwell's equations determine the relation between the measured field and object magnetization. The fields of several objects are added together linearly according to the superposition principle. The volumetric magnetic susceptibilities for biomaterials and contrast agents at practical concentrations are much smaller than one (<<100 ppm), and accordingly their mutual polarization effects may be ignored.
There are difficulties of quantification in molecular MRI using traditional relaxation susceptibility contrast mechanisms. For example, using superconducting detection coils, a superconducting quantum interference device (SQUID) can detect small flux of the magnetic field of an object magnetized by a primary field. Assuming the object is comprised of regions of uniform susceptibility distribution, regional susceptibilities are related to SQUID coil fluxes through numerically calculated geometry factors, allowing estimation by inverting a set of linear equations. It has also been proposed that a 3D susceptibility distribution may be reconstructed by using a composite of multiple SQUID coils in a manner similar to the inversion used in magnetoencephalography (MEG). Because SQUID coils of finite sizes have to be placed outside the human body, the number of flux detectors is limited, the inversion reconstruction is not well behaved, and the spatial resolution of mapping static susceptibility is very poor as demonstrated in MEG. The poor spatial resolution (˜1 cm) will make it difficult to resolve magnetic biomarker distribution in molecular imaging. Furthermore, this technology is not widely available and its clinical applicability is therefore limited.
MRI signal phase is proportional to magnetic field times echo time. Therefore, each voxel in MRI is analogous to a self-contained SQUID coil, allowing detection of local magnetic fields associated with the magnetization of susceptible materials polarized by B0 of the MR scanner. Since there are many voxels available for field detections, MRI can be a powerful method for determining susceptibility.
The static dephasing regime theory may be used to estimate susceptibility from signal amplitude temporal variation assuming a voxel containing many identical susceptibility particles and background materials. The application of this assumption in practice remains to be investigated. Furthermore, signal may not be measurable everywhere, for instance near regions of signal void caused by strongly varying susceptibility or low water density.
Currently, dark regions in T2* weighted MRI have been used to identify the presence of iron, by interpreting the observed signal loss as caused by intravoxel dephasing effects of local magnetic fields of iron deposits. This hypointensity depends on voxel size and orientation, may be confused with other signal voids, leading to inaccurate visualization and inadequate quantification of iron deposits.
Brain iron has been investigated substantially for a potential biomarker for many brain diseases including neurodegenerative diseases (Parkinson's disease, Alzheimer's disease, Huntington's disease, and multiple sclerosis), iron overloading disorders, chronic hemorrhage, cerebral infarction and other causes of bleeding and microbleeding. Localized iron in the brain is primarily in the forms of ferritin and its degradation product, hemosiderin. Ferritin has a large spherical protein coat (˜12 nm in diameter) that surrounds a crystalline core of hydrous ferric oxide. As much as 4500 iron atoms (Fe3+) can be stored in the 8-nm-diameter internal cavity of one ferritin protein. Hemosiderin appears to be associated with iron overload disorders and hemorrhage.